In general, a suspension system for use in a vehicle controls a damping force in relation to relative movements of a vehicle body and wheels. For example, the damping force variable type shock absorber including the damping force variable valve absorbs vibrations of the vehicle caused by irregularities of a road when the vehicle is in a normal drive mode, thereby improving ride comfort. Meanwhile, when the vehicle makes a turn, accelerates, brakes or runs at a high speed, the shock absorber increases the damping force and prevents a posture change of the vehicle body, to thereby improve stability in controlling the vehicle.
Since the movement of the wheels of the vehicle requires rapid response more than 10 Hz, there have been made various attempts to develop a valve capable of rapidly adjusting a damping force by means of a mechanical valve mechanism inside the shock absorber depending on the relative movements of the vehicle body and the wheels of the vehicle while independently controlling damping force characteristics during a compression and a rebound stroke.
Conventional variable valves for use in a suspension system are classified into two groups depending on the type of damping force control methods: normal type variable valve and reverse type variable valve. A reverse type variable valve controls the compression stroke and the rebound stroke by using separate valves incorporated therein depending on the movement of the vehicle. Thus, the reverse type variable valve serves to generate a small damping force during the rebound stroke while generating a great damping force during the compression stroke or vice versa. Since, however, the reverse type variable valve uses the separate valves, manufacturing costs are increased and the size of the reverse type variable valve is enlarged as well, resulting in a reduction of installation efficiency.
A normal type variable valve controls a damping force during both the compression stroke and the rebound stroke by using a single valve. Thus, the normal type variable valve serves to generate a great damping force or a small damping force in both of the rebound stroke and the compression stroke.
Referring to FIG. 1, there is provided a cross sectional view showing the configuration of a conventional variable type shock absorber 1. The interior of a cylinder 10 is divided into a rebound chamber 2 and a compression chamber 3 by a piston 11 which moves up and down inside the cylinder 10. Further, a piston rod 12 and a reservoir 13 are configured such that they are communicated with the cylinder 10, wherein one end of the piston rod 12 is connected to the piston 11 while the other end thereof is extended to the outside. The reservoir 13 serves to compensate a volume variation of the inside of the cylinder 10. Further, a valve 14 for allowing a flow of oil between the rebound chamber 2 and the compression chamber 3 is installed at the piston 11, and a valve 15 for allowing a flow of oil between the reservoir 13 and the compression chamber 3 is disposed at a bottom portion of the cylinder 10. Here, one or a plurality of check valve for allowing a flow of oil in a certain direction without generating a damping force and a damping valve for allowing a flow of oil in a certain direction while generating a damping force can be used as the valves 14 and 15. A damping force variable valve 20 is installed at one end of an outer diameter portion of a base shell 16 serving as a casing of the shock absorber 1.
FIG. 2 shows the configuration of a conventional normal type variable valve, which includes a fixed orifice 22, a control chamber 23, a variable orifice 24, a solenoid coil 26, a solenoid member 27, a spring 28, a valve member 29 and a valve seat 30, and so forth.
Assume that an acceleration is generated in a vertical direction of the suspensions system, such as left and right wheels of the vehicle or control arms (not shown), due to vibrations and driving state of the vehicle when the vehicle is running. In case of the conventional normal type variable valve configured as described, an acceleration detecting sensor (not shown) detects an acceleration and transmits a signal to an ECU (not shown) and the ECU investigates the received signal. Then, if electric current is allowed to flow to the solenoid coil 26, a magnetic field is generated, and a position of the solenoid member 27 provided at a central portion of the solenoid coil 26 is controlled by using the magnetic field.
A fluid is introduced into the control chamber 23 from a high pressure section 21 via the fixed orifice 22 and is introduced into a low pressure section 25 via the variable orifice 24. However, in case the pressure difference between the high pressure section 21 and the control chamber 23 is increased due to an increase of a flow rate of the fluid during a rebound or a compression stroke, the valve member 29 having been in a firm contact with the valve seat 30 by an elastic force of the spring 28 gets lifted from the valve seat 30. As a consequence, a main valve including the valve member 29 and the valve seat 30 is opened, so that the fluid becomes to flow from the high pressure section 21 to the low pressure section 25 directly without passing through the control chamber 23.
A point in time when the valve member 29 is lifted from the valve seat 30 is referred to as a blow-off time. The blow-off time is determined by a cross sectional area of the variable orifice 24. Specifically, when the electric current is flown to the solenoid coil 26, the position of the solenoid member 27 is determined by a magnetic force of the solenoid, whereby the cross sectional area of the variable orifice 24 is determined. Thus, by adjusting the cross sectional area of the variable orifice 24, the blow-off time when the main valve is opened can be controlled, so that the damping characteristics of the variable valve can be controlled.
Referring to FIG. 3, there is illustrated a flow passage diagram of the normal type damping force variable valve shown in FIG. 2. As shown therein, the conventional normal type damping force variable valve includes a first flow passage Qc having a fixed orifice Kc, a control chamber and a variable orifice Kv between a high pressure section Ph and a lower pressure section P1; and a second flow passage Qm having a main valve Km. Here, if the pressure difference between the high pressure section Ph and the control chamber is increased, the main valve Km is opened by overcoming the elastic force of the spring. That is, by controlling a pressure Pc of the control chamber by way of adjusting the cross sectional area of the variable orifice Kv provided downstream of the control chamber, a desired damping characteristic can be obtained.
Since an appropriate damping force should be generated within a range where a flow rate is great, i.e., where the piston moves at a high speed (hereinafter, referred to as a high speed range) in order to obtain an appropriate damping force characteristic, the blow-off time of the main valve needs to be obtained at a low flow rate and a high pressure in a hard mode (in which the cross sectional area of the variable orifice is reduced). For this, active area of the high pressure section of the main valve should be always greater than the active area of the control chamber in the structure of the conventional normal type damping force variable valve, which results in a complicated structure of the main valve. Moreover, since the pressure and the flow rate characteristics are controlled by the single variable orifice within a range where a flow rate is small, i.e., the piston moves at a low speed (hereinafter, referred to as a low-speed range), prior to the blow-off time, there is a drawback in that the tuning of damping force can not be performed independently within the low speed range in a soft mode and a hard mode.